1. Field of the Invention
This invention relates to friction welding of metal, in particular aluminium alloy components, and in particular those used in situations where high strength is required such as in structures for aircraft, helicopters, hovercraft, spacecraft, boats and ships.
2. Discussion of Prior Art
Structures and processes of the invention find particular application in aircraft structure, including primary structure, where strength to weight ratio is paramount.
Airframe structural components are inherently complex in their design and subsequent manufacture owing to the large variety of stresses which will be applied to the structure in different phases of aircraft operation, e.g. static, level flight, climb, descent, take-off and landing or gust conditions. In order to simplify and reduce the number of airframe components it is a well known principle to integrally machine from solid billets such components. In this way the parts counts and therefore the weight, cost and complexity of the finished assembly can be reduced. However limitations upon designs which are achievable currently exist owing to restrictions on manufacturing capabilities, for example in terms of overall billet size combined with the unavailability of welded joints for many primary aircraft structures owing to the well-known fatigue-inducing and crack propagation qualities of welded joints.
An example of current design limitations in aircraft wing manufacture occurs in the available size of upper or lower wing skin panels for construction of a wing box. At present, for large passenger-carrying aircraft such as the Airbus A340 family, certain areas of the wing box require a spliced joint between up to four separate machined panels where a single panel would be desirable. The overall weight and cost of wing skins formed by the panels is increased. Also a single panel to replace the multi-panel assembly would be structurally more efficient. The present limitation on panel size is caused by a limitation on size of the aluminium alloy billet from which the panel is rolled.
A further example of the limitations imposed by present technology occurs in the manufacture of solid aluminium alloy billets from which inner wing spars are formed for large commercial aircraft. Any increase in size of such aircraft, as is presently projected for a future large passenger-carrying aircraft would result in a requirement for a billet larger than it is currently possible to produce. This restriction raises the need for complex bolted joints between components. Such joints will considerably increase the weight and complexity of the structure and will be structurally non-optimum.
Design difficulties can also occur at the intersections between upper and lower wing skins and upper and lower spar flanges respectively in an aircraft wing box. Upper and lower wing skins will be made of different alloys to enable the different structural requirements to be fulfilled. Where these different alloys are joined to the wing spar fatigue cracking can occur owing to the differing material properties of the skin and spar respectively.
Yet further difficulties can occur in achieving an optimum cross sectional shape at acceptable cost for extruded aircraft wing skin stiffeners, for example stringers. Here the additional material required at the ends of the stringers, often called for example “spade ends” or “rib growouts” can dictate the sectional shape for the whole length of the stringer and can necessitate machining off unwanted material for almost the entire length of the stringer, leading to excessively high machining and material scrap costs.
According to one aspect of the invention there is provided a method of forming a structural airframe component for an aircraft including placing at least two components in abutting relationship with each other and joining them together by friction stir butt welding.
The structural airframe component may comprise an aircraft wing rib and the at least two components may comprise a central web element and a rib foot element and the method may include the steps of joining together the central web element and the rib foot element by partial penetration friction stir butt welding and subsequently machining away material from at least one of the central web element and the rib foot element in the region of the abutment until the weld becomes a full penetration weld.
The method may also include the steps of providing a said rib foot element of L-shape cross section and carrying out the machining away of material at least from the rib foot element to form a rib foot of T-shape cross section.
“Butt welding” as used herein is intended to include the process of welding together at least two components having edges or surfaces in abutment with each other, whether the components are generally co-planar in the region of abutment or not.
The technique of friction stir butt welding is known from European Patent No. 615480B assigned to The Welding Institute the entire contents of which are incorporated herein by reference. The technique involves placing the two said components in abutting relationship with each other, inserting a probe of material harder than the component material into a joint region between the two components and causing relative cyclic movement between the probe and components whereby frictional heat is generated to cause portions of the components in the region of the joint to take up a plasticised condition, removing the probe and allowing the plasticised portions to solidify and join the components together.
The application of this technique to aircraft airframe structure, including primary load bearing structure would not have been foreseen owing to the aforesaid known properties of welds, namely liability to fatigue. Surprisingly however work carried out has revealed that such friction stir butt welds do indeed possess the qualities to make such structures as aforesaid possible.
In order to exclude the possibility of cracks developing in the region of the weld joint, a weld fatigue resistant feature may be applied to a run-out of the weld. Such a feature may comprise a cold worked hole formed through the weld joint in the region of the run-out followed by insertion of a fastener, for example a bolt. Alternatively, or in addition the joined component in the region of the weld run-out may be shot peened or may have a splice strap fastened in position transverse to the direction of the weld joint. Further in the alternative or additionally the material of the welded component in the region of the weld run-out may be thickened. By the various above means one of the primary areas of fatigue of the welded joint may be prevented from behaving in such an adverse manner.
The friction stir butt welding method may be applied to components having a differing, for example tapering, thickness of material to be welded together by inserting the said probe into the joint between the two components to a depth dependent upon the material thickness at the position of probe entry. In this way a weld having a penetration through the material of the component of sufficient depth to provide a prescribed weld penetration along a length of the weld may be achieved.
The method may include providing a two-piece probe having a central portion for penetrating the weld region and a peripheral portion movable relative thereto for travelling over the weld region along surfaces of the components being joined, the central portion being movable into and out of the peripheral portion during welding. The central portion and peripheral portion may be relatively movable by a threaded connection therebetween or by any other suitable mechanism like a geared connection or a cam means. The central portion and peripheral portion may include sealing means acting therebetween to prevent ingress of softened component material.
During movement along the weld of a said component of varying thickness such as a tapered component the rate of feed of the probe along the joint and rotational speed of the probe may be varied to optimise welding conditions.
The structural airframe component for an aircraft may comprise a skin stiffener and the method may include the step of placing an extrusion in abutting relationship with an extension or width-increasing region for the extrusion and joining the extrusion to the extension region by friction stir butt welding. In this way extension regions such as rib growouts and stringer spade ends and other root end profiles of width larger than the extruded width may be formed on the stiffener without resorting to the formation of an extrusion of the maximum width required and then machining long lengths of it away to leave just short lengths of the extension regions of the extruded width.
According to a second aspect of the invention there is provided a structural airframe component for an aircraft including a friction stir butt welded joint.
The component in the region of the butt welded joint may be double curvature in form. Also, the weld may be of tapering thickness along its length.
The component may comprise at least two skin panels butt welded together. Wing or fuselage skin or skin/stiffener panels of any required size can thus be produced according to the invention. Said skin panels may have a skin stiffener attached thereto along a length of the weld. Such a skin stiffener may comprise a skin stringer in which skin engaging flanges thereof are attached to the skin on either side of the weld.
The component may include a stiffened aircraft wing skin assembly comprising at least two extruded sections each having integrally formed skin-forming and stiffener-forming portions said sections being welded together. Each weld may include a butt strap fastened thereacross and may include a run-out feature as described above.
The component may comprise an aircraft skin and stiffener assembly including a joint between two stiffeners at which the skin is friction stir butt welded together.
The component may comprise an aircraft skin panel having a shaped plug friction stir butt welded into place therein. In this way local additions to or thickening of the skin during skin formation may be avoided.
The component may include a friction stir butt welded joint between two sub-components of differing cross section.
The component may comprise a hybrid billet of aluminium alloy comprising for example 7000 series alloy friction stir butt welded to 2000 series alloy to enable the known properties of these two alloys better to be employed. In general, friction stir butt welding of hybrid billets enables tailoring of thicknesses, material strengths and fatigue resistances as required. The billets may be for example forgings or extrusions according to the circumstances.
Where a friction stir butt welded joint replaces a joint using fasteners, as for example in the manufacture of large aircraft wing spars, there will be a reduction in the number of fasteners used overall, with a consequent reduction in cost and weight. Also the welded joint itself will be structurally more efficient in that it will be stronger than a fastened joint and, we have discovered, have better fatigue characteristics. In addition assembly time for a welded joint can be reduced, owing to elimination of fasteners and joint sealant. Also fuel leakage paths, in a wing, are eliminated.
The component may comprise an aircraft wing rib or spar machined from a said hybrid billet. The hybrid billet may comprise two or more said friction stir butt welded joints positioned on the billet to optimise strength properties for the billet in the particular circumstances required. For example a said spar may include a weld along a neutral axis thereof or may include such welds at or in the region of junctions between a central web and upper and lower booms thereof.
The component may comprise an I-section or J-section stiffener of web height tapering along the length of the stiffener having a friction stir butt welded joint extending along the length of the tapered web.
The component may comprise an aircraft skin panel having at least one first part stiffener formed integral therewith with a further part of the stiffener friction stir butt welded to the first part of the stiffener.
The component may comprise an extruded aircraft skin stiffener including at least one extension region thereof extending the width of the stiffener beyond an extruded width, said at least one extension region being attached to the remainder of the stiffener by a friction stir butt welded joint. The at least one said extension region may comprise at least part of a rib growout or a spade end or other root end profile region of a skin stringer.
The component may comprise an I-section or a J-section skin stiffener having upper and lower booms or flanges separated by a central web and the at least one extension region may be friction stir butt welded to at least one of the upper and lower booms on one or both sides of the web.